The technical field of this invention is vehicle traction control.
A known traction control for a motor vehicle calculates a target velocity for a slipping drive wheel relative to a reference speed of the vehicle and applies an individual braking command to the wheel and/or decreases engine power to bring the speed of the slipping wheel toward the target velocity. The target velocity is derived as the sum of the vehicle reference speed and a target delta velocity, the size of which controls the desired amount of wheel slip. The target velocity is derived in a manner to optimize vehicle acceleration, stability and steerability. It is calculated as a function of longitudinal acceleration and vehicle speed, lateral acceleration and turn curvature, with the contribution of each of these parameters optimized for a specific vehicle application. The target delta velocity is then bounded between maximum and minimum values that define an optimal range of target delta velocities to optimize vehicle performance for a variety of surfaces and vehicle operating conditions.
During evasive or performance driving on low or medium coefficient of friction surfaces, a vehicle stability enhancement (VSE) system may be activated to improve driver control. The VSE system applies the vehicle brakes or some other wheel slip affecting system in response to a detected error between a measured vehicle yaw rate and a desired yaw rate calculated from vehicle handling parameters such as vehicle speed and steering angle.
The method and apparatus of this invention provides traction control for the drive wheels of a motor vehicle that supplements the action of a vehicle stability enhancement system during vehicle stability enhancement events by increasing and/or decreasing slip of individual vehicle drive wheels to complement the braking or similar action of the VSE system.
The method and apparatus of this invention provides traction control of the drive wheels of a motor vehicle by deriving a target velocity from a vehicle longitudinal speed and one or more of a vehicle longitudinal acceleration, a vehicle lateral velocity and a vehicle turn curvature, estimating a coefficient of friction between the vehicle wheels and the drive surface, and, responsive to activity of the vehicle. stability enhancement system in reducing a vehicle yaw rate error, bounding the target velocity between a maximum target velocity value and a minimum target velocity value, with at least one of the maximum target velocity value and the minimum target velocity value being derived from the estimated coefficient of friction. Preferably, the one of the maximum target velocity value and minimum target velocity value that is derived from the estimated coefficient of friction is greater at a high magnitude of the estimated coefficient of friction than at a low magnitude of the estimated coefficient of friction. In a preferred embodiment, the understeer/oversteer behavior of the vehicle is derived, for example by the vehicle stability enhancement system. Preferably, if the drive wheels are front wheels of the vehicle, the maximum target velocity value is derived from the estimated coefficient of friction when the vehicle is in understeer and the minimum target velocity value is derived from the estimated coefficient of friction when the vehicle is in oversteer. Preferably, if the drive wheels are the rear wheels of the vehicle, the maximum target velocity value is derived from the estimated coefficient of friction when the vehicle is in oversteer and the minimum target velocity value is derived from the estimated coefficient of friction when the vehicle is in understeer.